This invention relates to a heat roller fixing apparatus. In an electrophotographic copying machine or an electrostatic recording machine, a fixing apparatus for fixing images on a sheet or supporting material is incorporated. One example of such apparatus is a fixing roller for heat fusing the image.
In one conventional heat fixing apparatus, a pair of fixing rollers is provided each having a chamber containing a heat transfer or heating medium, which is sealed therein and connected to saturated vapor by a heater incorporated within a cylinder inside the roller. The rollers are disposed in contact with each other and a sheet or supporting material having an image to be fixed thereon is fed between them and heated by the rollers so that the image is fixed by heat fusing.
In the above-mentioned fixing rollers, the temperatures of the surfaces of the respective rollers are detected by a temperature detecting means, such as a thermistor, disposed on the surfaces of the respective rollers, and a necessary command is given to control means for the heater so that the fixing temperature of the respective roller surfaces is determined. This sort of fixing apparatus, however, has the shortcoming that the heating medium sealed in the chamber of the fixing roller is small in quantity as shown in FIG. 1. As a result such a small quantity of heat transfer medium lies at the bottom of each chamber, and its upper surface does not come in contact with the inner cylinder holding the heater therein when the respective rollers are stationary. Therefore, when the heaters are energized while the rollers are stationary, the respective inner cylinders are heated, but heat is not readily transferred to the heating medium, and accordingly not to the surface of the respective rollers as the heating medium is not in contact with the inner cylinder. Therefore, the temperature detecting means, which is disposed in contact with the surface of the respective rollers, does not detect the accurate temperature of the heat to be transferred from the inner cylinder, and it gives the control means of the heater a wrong command demanding more heat. As a result, the inner cylinder is heated excessively, causing a very dangerous situation of damaging the inner cylinder.
In order to prevent the above-mentioned accident, conventionally the following apparatus have been devised: apparatus wherein a detecting means, which detects the rotation of the heating roller, is incorporated whereby the heater is energized only when the heating roller is being rotated; and apparatus wherein blades are attached to the inner cylinder which scoop up the heat conductive medium as the inner cylinder rotates as shown in FIG. 1.
However the former apparatus has the shortcomings that the detecting means is not reliable and the apparatus is expensive while the latter apparatus is intricate in mechanism. Therefore, investigations to remove the above-mentioned problems from the heat fixing roller have been necessitated.